The present invention relates generally to optical devices and, more particularly, to an optical device including an arrangement configured for producing simultaneous orthogonal walk-offs of light polarization components. The invention enjoys applicability in any device requiring routing of orthogonally polarized light signal components traveling at least at one point along two different light paths.
State of the art communications/data transmission systems increasingly rely upon the transfer of information in the form of light signals. These light signals are typically transmitted through optical fibers. Consequently, needs have arisen for interfacing with the optical fibers and for manipulating the light signals in certain ways outside of the fibers and returning the light signals to the fibers. In order to satisfy both of these needs, various functional types of optical devices have been developed. Such optical devices include, for example, optical circulators, optical switches and filters including add, drop and drop-add configurations.
One example of a prior art optical circulator is shown in FIG. 1. The latter is a reproduction of FIG. 2a taken from U.S. Pat. No. 5,574,596 showing an optical circulator generally indicated by the reference numeral 10. Initially, it should be noted that circulator 10 is formed from various layers of crystalline material. The circulator includes an input arrangement 12 and an identical output arrangement 14. A polarization shifting arrangement 16 is positioned between the input and output arrangements and comprises first and second birefringent walkoff crystals 18a and 18b which may be referred to hereinafter as a birefringent crystal pair. During operation, optical circulation is provided between three ports, as indicated by the reference numbers 20a, 20b and 20c, respectively. It should be noted that an interface surface 22 is defined by the birefringent crystal pair.
Referring to FIG. 2 in conjunction with FIG. 1, walkoff crystal 18a includes a first walkoff direction 24a while walkoff crystal 18b includes a second walkoff direction 24b. In this regard, it should be noted that the first and second walkoff directions oppose one another and are generally parallel with interface surface 22. Moreover, it should also be noted that other devices have been seen in the prior art which possess crystal pairs forming interface surfaces in the manner illustrated by FIG. 1. See, for example, U.S. Pat. No. 5,204,771 which discloses a polarization rotation crystal pair.
Referring again to FIG. 1, it is submitted that a particular disadvantage is associated with circulator 10. Specifically, an unfocused beam of light injected into any of ports 20 must initially pass through either input arrangement 12 or output arrangement 14. In either case, the beam will exhibit expansion by the time it reaches the birefringent crystal pair. Further beam expansion will then occur as the beam passes through the birefringent crystal pair. Unfortunately, polarization components of the beam are likely to interact with interface 22 as a result of the beam expansion. The result of this interaction is signal "clipping" wherein a portion of the beam power is undesirably lost. Therefore, it is submitted that any practical implementation of the device shown in FIG. 1 will require external focusing components (not shown) such as, for example, lenses which serve to focus light emitted by fiber optic cables into the circulator body in compensation for beam expansion. However, it should be appreciated that the lenses required for this focusing necessitate an undesirably large beam separation. That is, the lateral distance between ports 20a and 20b must be large enough to accommodate the lenses. The beam separation necessitated by the lenses itself establishes the amount of walkoff which must be produced by walkoff elements within the overall assembly with respect to walking beams or components thereof to a common path from two different paths. Therefore, the use of focusing lenses leads to a direct increase in the length of the walkoff elements required. In this regard, it should be appreciated that such an increase in the length of the walkoff elements also increases manufacturing costs since the cost of such elements is strongly related to their volume.
Still considering the circulator of FIG. 1, one advantage associated therewith, as recognized by Applicant, is worthy of mention for purposes of later reference. In particular, the device may be expanded to accommodate additional light signals and/or circulator configurations by simple extension of the lateral extents of the device along the direction defined between ports 20a and 20c.
Turning now to FIG. 3, a more recent class of optical devices has been developed as indicated by reference number 30. FIG. 3 is a partial representation of FIG. 1E from U.S. Pat. No. 5,734,763. Device 30 includes a front end 32 in optical communication with a device body 34. Front end 32 includes first and second birefringent crystal pairs 34 and 36. Crystal pair 34 includes first and second birefringent crystals 38a and 38b which are in physical contact in the overall final assembly of the device whereby to form a first interface surface/region as defined by adjacent surfaces 40a and 40b. The first and second crystals further include first and second walkoff directions which are parallel to surfaces 40a and 40b in opposing first and second directions as indicated by arrows 42a and 42b, respectively. Second crystal pair 36 includes third and fourth birefringent crystals 46a and 46b which, like the first crystal pair, are in physical contact in the overall final assembly of the device so as to form a second interface surface/region as defined by adjacent surfaces 48a and 48b. The third and fourth crystals include third and fourth walkoff directions indicated by arrows 49a and 49b that are parallel to surfaces 48a and 48b. During operation, light may be emitted into or received from the device as constrained by the presence of the interface surfaces. That is, any light signal which passes through front end 32 will experience clipping if the signal impinges on either of the interface surfaces. It should be appreciated that the first and second interface surfaces are in an orthogonal orientation with respect to one another such that front end 32 is, in essence, divided into four quadrants (not specifically indicated in the Figure). Therefore, in order to avoid clipping, as it passes through front end 32 a signal must remain sufficiently far away from the interface surfaces. As an example of ports on the front end, in the instance of a circulator, port 1 may be defined, as indicated by reference number 50, at a suitable location on crystal 38b for one quadrant while port 3 may be defined, as indicated by reference number 52, at a suitable location on crystal 38a for an orthogonally opposed quadrant.
While the device of FIG. 3 is well suited for its intended purpose, it should be appreciated that the device is not linearly expandable in the manner described above with regard to circulator 10 of FIG. 1. That is, the orthogonal arrangement of the interface surfaces defined by the first and second birefringent crystal pairs removes the potential for linear expansion of the device by simple expansion of the dimensions of the device along one axis, as may be done in the instance of the device described in FIG. 1. One of ordinary skill in the art may suggest that the device of FIG. 3 will exhibit a problem similar to that of the device of FIG. 1 with regard to signal clipping. However, the clipping problem is alleviated in the FIG. 3 device primarily due to the fact that front end 32 itself defines the ports, at least at one end of the device, such that the signals pass the interface surfaces before significant beam expansion can take place. That is, relatively low levels of beam expansion are present at the front end where the interface surfaces are located. For the same reason, the need for input lenses is thought to be reduced in device 30 of FIG. 3. Thus, the devices of both FIGS. 1 and 3 exhibit significant, yet independent problems and advantages.
The present invention provides a class of optical devices which serve to eliminate the problems described above with regard to FIGS. 1 through 3 in a heretofore unseen and highly advantageous way. That is, the disclosed devices couple the advantage of avoiding signal clipping at crystal interface surfaces in compact package with the advantage of providing linear expansion of the device along a single dimension for the purpose of processing additional light signals in a cost effective manner.